Amplifiers for supplying current at power levels to a load are extensively used. For example, in magnetic disk drives, the magnetic read/write head is positioned by an actuator driven by a voice coil motor. U.S. Pat. Nos. 4,439,800-Powell and 4,620,252 - Bauck, et al show an actuator for a magnetic disk drive. Such actuators must be driven with power current proportional to an input signal. This is difficult because the voice coil has a high inductance and it is difficult to drive the coil with a high current having a fast rise time. Bipolar power operational amplifiers are typically used to drive these actuators but improvement in input signal tracking is desirable.
Newly developed field effect transistors called sensfets have been used to drive motors and the like. Such devices are described in MACHINE DESIGN, Jun. 23, 1988, pp. 100-105. In addition to the normal source, drain and gate elements of a field effect transistor, the sensfets also have a mirror pin for load current sensing and a Kelvin pin for temperature compensation. The mirror pin current is a small linearly proportional fraction of the track load current between the source and the drain. This provides sensing output which is used to obtain essentially lossless load current sensing. These devices have been extensively used in circuits which turn the current on or off. For example, in pulse width modulation circuits sensfets turn the load current on or off.